The Transcallosal Highway: The ipsilateral silent period as a neural biomarker for impaired corpus callosum communication in multiple sclerosis

Abstract

Multiple sclerosis is a neurodegenerative disease that damages myelin sheath within the central nervous system. This axonal demyelination impacts communication between the brain's hemispheres in persons with multiple sclerosis (PwMS). Changes in transcallosal communication impairs the coordination of movements requiring precise temporal and spatial components such as lower extremity function during gait. These bilateral movements require constant communication across the corpus callosum to excite and inhibit specific muscle groups. The ipsilateral silent period (iSP) is an indirect marker of the magnitude for transcallosal inhibition. We hypothesize that the iSP may also serve as a neural biomarker for transcallosal impairments originating from the more affected hemisphere and highlight underlying mechanisms for gait asymmetries in PwMS. Our ongoing study utilizes transcranial magnetic stimulation to assess the inhibitory capacity between the brain's hemispheres (i.e., iSPs). A focal magnetic pulse is delivered to the first dorsal interosseous resulting in a suppression of muscle activity, thus reflecting inhibitory transcallosal communication. There is a lack of research analyzing directionality data between the more and less affected hemisphere in PwMS. Therefore, we evaluate outcome metrics dependent upon the individual's more affected hemisphere. Twenty-three PwMS completed the ongoing protocol and inhibitory metrics such as depth iSP% average, duration, depth iSP% max, transcallosal conduction time, and onset latency were collected. No statistically significant differences have been found between the two hemispheres in PwMS. However, beyond directionality data, our study is investigating gait coordination with overall inhibitory capacity. Walking coordination is quantified using a metric called phase coordination index (PCI). A greater PCI value reflects poorer gait coordination. We hypothesize those who demonstrate better gait coordination (lower PCI values), will have a greater iSP. These findings will determine the potential of iSPs as a neural biomarker to address gait asymmetries and stratify participants into individualized mobility rehabilitation protocols. Multiple sclerosis is a neurodegenerative disease that damages myelin sheath within the central nervous system. This axonal demyelination impacts communication between the brain's hemispheres in persons with multiple sclerosis (PwMS). Changes in transcallosal communication impairs the coordination of movements requiring precise temporal and spatial components such as lower extremity function during gait. These bilateral movements require constant communication across the corpus callosum to excite and inhibit specific muscle groups. The ipsilateral silent period (iSP) is an indirect marker of the magnitude for transcallosal inhibition. We hypothesize that the iSP may also serve as a neural biomarker for transcallosal impairments originating from the more affected hemisphere and highlight underlying mechanisms for gait asymmetries in PwMS. Our ongoing study utilizes transcranial magnetic stimulation to assess the inhibitory capacity between the brain's hemispheres (i.e., iSPs). A focal magnetic pulse is delivered to the first dorsal interosseous resulting in a suppression of muscle activity, thus reflecting inhibitory transcallosal communication. There is a lack of research analyzing directionality data between the more and less affected hemisphere in PwMS. Therefore, we evaluate outcome metrics dependent upon the individual's more affected hemisphere. Twenty-three PwMS completed the ongoing protocol and inhibitory metrics such as depth iSP% average, duration, depth iSP% max, transcallosal conduction time, and onset latency were collected. No statistically significant differences have been found between the two hemispheres in PwMS. However, beyond directionality data, our study is investigating gait coordination with overall inhibitory capacity. Walking coordination is quantified using a metric called phase coordination index (PCI). A greater PCI value reflects poorer gait coordination. We hypothesize those who demonstrate better gait coordination (lower PCI values), will have a greater iSP. These findings will determine the potential of iSPs as a neural biomarker to address gait asymmetries and stratify participants into individualized mobility rehabilitation protocols.